Inter-cell interference coordination (ICIC) was introduced in Release-8/9 of the 3GPP LTE standards. The basic idea of ICIC is keeping the inter-cell interferences under control by radio resource management (RRM) methods. ICIC is inherently a multi-cell RRM function that needs to take into account information (e.g. the resource usage status and traffic load situation) from multiple cells. Broadly speaking, the main target of any ICIC strategy is to determine the resources (bandwidth and power) at each cell at any time. Then (and typically), a scheduler assigns those resources to users. Static ICIC schemes are attractive for operators since the complexity of their deployment is very low and there is no need for new extra signaling out of the standard. In WCDMA and LTE systems, one-cell frequency reuse deployment is applied, where the entire frequency spectrum can be allocated to a cell and its neighboring cells. Thus, static ICIC mostly relies on the fractional frequency reuse concept, where the total system bandwidth is divided into sub-bands and used by the scheduler accordingly.
LTE Release-8/9 ICIC techniques, i.e., FDM-based ICIC, are not fully effective in mitigating control channel interference. For example, dominant interference condition has been shown when non-CSG (close subscriber group) macrocell users are in close proximity of CSG femtocells. Therefore, enhanced ICIC (eICIC, also referred to as TDM ICIC) has been investigated from Release-10 onwards to provide enhanced interference management. In LTE/LTE-A Release-10, two main inter-cell interference scenarios for eICIC were being discussed: Macro-Pico scenario and Macro-Femto scenario. In general, almost-blank subframe (ABS) or silenced subframe concept is introduced to reduce inter-cell interference. When ABS is applied, the aggressor cell suspends the scheduling or transmits with smaller power so that the victim cell can conduct data transmission in the protected subframes.
In Macro-Pico scenario, a network with macrocells and picocells on the same or overlapping carrier frequency, where picocells are re-expanded to offload more traffic from the macrocells. In this scenario, a macrocell is the aggressor and may introduce strong interferences to picocells, which are called victim cells. ABS is applied in the macrocell so that UEs can try to search for picocells in the protected subframes and to maintain connection at the cell edge of the picocell. In Macro-Femto scenario, a network with non-accessible CSG femtocells deployed on the same or overlapping carrier frequency as macrocells. In this scenario, victims UEs are connected to a macrocell but in the coverage of a femtocell, but cannot be handed over to the femtocell because of non-CGS membership. The femtocell is the aggressor cell and implements a pattern of silenced subframes. The macrocell is the victim cell and makes use of the silenced subframes for the UEs that are in highly interfered situation.
In LTE/LTE-A systems, one radio resource management (RRM) scheme is that the UE may report measurement results to its serving base station (eNB) for better scheduling and mobility management. When eICIC or TDM ICIC is applied, it has been discussed that UE measurements on victim cells shall take place in the silent periods of a silencing pattern. This is especially true for UEs that are served by the victim cell in order to maintain connection to the cell. This is also true when a neighbor cell is the victim cell, otherwise handover to an interfered victim cell would not be possible. In current LTE/LTE-A design, the above measurement behavior is applied in the intra-frequency measurements. For example, a set of TDM measurement restriction is attached to a measurement object in a frequency layer.
For inter-frequency measurements, however, there is no agreement that whether an explicit TDM measurement restriction is needed for UE measurement of neighbor aggressor cells that implement TDM ICIC silencing. In general, without measurement restriction, the RSRQ (reference signal received quality) measurement has high un-predictability when UEs are measuring an aggressor cell that applies TDM ICIC silencing. This is because the RSRQ measurement results depend on which subframes the UE happens to select for measurements. RSRQ is defined by RSRP/RSSI, where RSRP is Reference Signal Received Power and RSSI is Received Signal Strength Indicator. In TDM eICIC measurement, the RSSI value varies among subframes because some of them are sliced subframes, and thus causes RSRQ value fluctuation. As RSRQ is used for mobility, especially for inter-frequency mobility, RSRQ fluctuation could lead to unpredictable mobility behavior and ping-pong effect etc. For example, the RSRQ measurement on a victim cell could be over pessimistic when measurement takes place in the non-protected subframes. Furthermore, it could be expected that UEs from different vendors could give different results.
On the other hand, if measurement restriction is needed for inter-frequency measurements when TDM eICIC is applied, then potentially many measurement restrictions may have to be configured in the UE, which may lead to high complexity in the UE. Another problem is that for inter-frequency scenario, when RSRQ measurement is performed with measurement gaps, the resulting measurement opportunities from combining measurement gaps and TDM ICIC restrictions may be too few, which may cause larger impact. Even if appropriate measurement restriction were agreed for RSRQ measurement, the UEs would still experience large fluctuations in RSRQ measurement results because of too few measurement samples. It is thus an objective of the current invention to improve the RSRQ measurement predictability without requiring an explicit TDM measurement restriction.